This invention relates to anisotropic cellulose solutions and to fibers and films produced therefrom. More specifically, the present invention relates to the production of anisotropic cellulose solutions in the nematic phase as the preferred precursor to high strength, high modulus cellulose fibers and films.
Anisotropic spinning solutions have been recognized as being useful for forming fibers and films of high tenacity and modulus. For example, anisotropic polyamide spinning solutions such as poly(p-phenylene terephthalamide) have been employed in producing high tenacity aramide fibers known commercially as Kevlar.RTM. fibers, as disclosed for example in Kwolek U.S. Pat. Nos. 3,671,542, 3,819,587, and Re 30,352.
It has also been observed that cellulose and cellulose derivatives can form anisotropic solutions. Chanzy.sup.1, Gilbert.sup.2, and Conio.sup.3 have reported that such anisotropic solutions could also be obtained from cellulose in various solvents. Gray4 lists a number of cellulose derivatives and solvent systems which have been reported to form anisotropic cellulose derivative solutions. References: FNT (1) H. Chanzy and A. Peguy, J. Polym. Sci., Polym. Phys. Ed., 18, 1137 (1980). FNT (2) D. L. Patel and R. D. Gilbert, J. Polym. Sci., Polym. Phys. Ed., 19, 1231 (1981). FNT (3) C. Conio, P. Corrazza, E. Vianchi, A. Tealdi and A. Ciferri, J. Polym. Sci., Polym. Lett. Ed., 22, 273 (1984). FNT (4) Gray, Derek G., Liquid Crystalline Cellulose Derivatives, Journal of Applied Polymer Science: Applied Polymer Symposium, 37, 179-192 (1983).
Anisotropic cellulose derivative solutions have also been reported to form high tenacity, high modulus cellulosic fibers. See for example Panar and Willcox French patent No. 2,340,344; Kamide, et al U.S. Pat. No. 4,370,168; and O'Brien U.S. Pat. Nos. 4,464,323 and 4,501,886.
While it has been shown, as noted above, that anisotropic solutions of cellulose are capable of forming high performance cellulose fibers, insofar as we are aware this technology has not been put into practice for producing cellulosic fibers on a commercial scale. This is believed to be attributable, at least in part, to certain limitations and disadvantages of the known cellulose/solvent systems. In certain of the known systems, anisotropic solutions containing the nematic phase are difficult to form (often requiring external stimuli such as magnetic fields). Further disadvantages or limitations of the known systems are lack of sufficient cellulose solubility to obtain wholly anisotropic solutions and the necessity of a pretreatment of the cellulose to activate it prior to its dissolution in the LiCl/DMAC system. In the case of the N-methylmorpholine-N-oxide (NMMO) solvent there is the problem of dealing with a solid solvent at room temperature and the need to melt it and carefully control its temperature. Further, the fact that NMMO is an amine oxide requires extreme safety precautions in its handling during extrusion and especially during recovery and recycling. The significant degree of polymerization (DP) loss of the cellulose at the spinning temperatures is another limitation.
The present invention, in one of its aspects, employs an advantageous solvent system based upon ammonia and ammonium thiocyanate, which readily forms anisotropic cellulose solutions which are stable and maintain anisotropic properties over extended periods of time.
Cuculo et al in U.S. Pat. No. 4,367,191, issued Jan. 4, 1983, have previously disclosed that ammonia/ammonium thiocyanate is an effective solvent for cellulose. The solvent dissolves high degree of polymerization cellulose to form solutions of high concentrations. The solvent is convenient, easy to prepare, inexpensive, safe, non-degrading towards cellulose and easily used at room temperature. Also, the cellulose requires no pretreatment for dissolution. The cellulose that is dissolved by this system may come from practically any source including cotton, wood pulp, and the dissolving pulp normally used by rayon producers.
In accordance with the Cuculo et al patent, the disclosed solutions of cellulose in ammonia/ammonium thiocyanate solvent were isotropic. These isotropic cellulose solutions were wet spun to produce cellulose fibers having properties essentially equivalent to conventional rayon.